1. Field of the Invention
The present invention relates to a laser ablative recording material, and in particular to a laser ablative recording material in which its over coat layer contains infrared-absorbing material having absorption in the laser wavelength region to achieve an improved sensitivity. The present invention also relates to a recorded matter containing an image formed with the laser ablative recording material.
2. Related Art
Recently, a thermal transfer system forming an image by imparting an electric signal to a thermal print head has become more popular. A method of forming an image by the use of a laser in place of the thermal print head was on the other hand developed, and is expected to become more popular along with the tendency toward a higher laser output.
A recording material for laser recording contains a material having a strong absorption in the laser wavelength region, and this absorbing material converts optical energy into thermal energy, and brings about effects similar to those available by the use of a thermal print head. Use of a laser, unlike the use of a thermal print head, permits heating without contact with a recording material, thus providing an advantage of the image surface free from flaws. Because of the possibility to stop down a laser beam, there is provided another advantage of improving image resolution.
A method for forming an image using a high-output laser known as the dye ablation has recently been developed. Japanese Unexamined Patent Publications Nos. 7-164,755, 7-149,063, and 7-149,065 (corresponding to U.S. Pat. No. 5,330,876, U.S. Pat. No. 5,401,618 and U.S. Pat. No. 5,459,017) disclose recording materials applicable in this method, and Japanese Unexamined Patent Publications Nos. 8-48,053 and 8-72,400 (corresponding to U.S. Pat. No. 5,521,629 and U.S. Pat. No. 5,574,493) disclose imaging apparatuses used in this method. Image recording based on the ablation method is accomplished by irradiating a laser from a dye layer side onto a recording material having a dye layer comprising an image dye, a material having absorption in the laser wavelength region (infrared-absorbing material) and a binder formed on a support. On the spot to which the laser beam has been irradiated, a sharp local change takes place in an image forming layer under the effect of energy from the laser, and this drives away the material from the layer. According to the aforesaid patent publications, this local change is not a perfectly physical change such as melting, evaporation or sublimation, but a kind of chemical change such as bond-breaking, and is believed to be a complete, not partial, removal of the image dye.
Usefulness of this dye ablation imaging method largely depends upon removal efficiency of the imaging dyes upon laser exposure. As a scale representing this efficiency, the minimum density value (Dmin) of the laser exposure portion is employed, where a smaller value of Dmin corresponds to a higher dye removing efficiency.
Thus has been proposed several methods for lowering Dmin to achieve improved sensitivity of the recording material. Japanese Unexamined Patent Publication No.7-149063, for example, describes use of a cyanine dye, having a specific zwitter ion, as a material which exhibits an intense absorption in the laser wavelength region. Japanese Unexamined Patent Publication No. 7-164755 specifies molecular weight of a binder as contained in a coloring agent layer. Japanese Unexamined Patent Publication Nos. 7-149065 and 8-52948 describe an intermediate layer (barrier layer) made of hydrophilic or hydrophobic polymer binder and interposed by coating between a support and a coloring agent layer. Japanese Unexamined Patent Publication No. 7-149066 discloses that Dmin can be improved by incorporating an infrared-absorbing material into the intermediate layer.
It has however been still difficult to obtain high sensitivity,high resolution and fully satisfactory image despite all improving efforts in photo-thermal conversion material (infrared-absorbing material), binder resin, image forming material and so forth.
It is therefore an object of this invention to solve the above problems. That is, it is an object of this invention to provide a laser ablative recording material capable of yielding image with a high sensitivity and high resolution. In other words, it is an object of this invention to provide a laser ablative recording material capable of achieving Dmin lower than that for any known ablative recording material without impairing the resolution. It is also an object of this invention to provide a recorded matter with a high resolution obtained by laser ablation.